We have a particular problem of conductor adhesion. We are extruding PVC insulation on Fibre Glass / Polyester braided conductor. The adhesion of the insulation with this braid is very poor. We are not able to do any pre-heating on the braided conductor. How can the adhesion of the Insulation with the FG/Polyester braid be improved?

First: What is the configuration of the extruder crosshead tip and die?
1) Are you tubing with vacuum pull-down?
2) Are you semi-tubing with vacuum pull-down or with no vacuum?
3) Are you pressure driving the PVC insulation on to the fiberglass/ polyester braided conductor?

Second: Is the PVC a flexible grade or is it semi-rigid?

Third: What is the line speed?

Fourth: Are you cooling the screw with water?

Fifth: Are you running the PVC hot?

Sixth: What is the % coverage of the braid? In other words is there actually room for the molten PVC to get between the sets of fibers in the fiberglass/ polyester braid?

Ans) - PVC is being dehumidified at aroud 100 degC and then fed in the hopper at around 50-60 degC.

Q.6 : What is the % coverage of the braid? In other words is there actually room for the molten PVC to get between the sets of fibers in the fiberglass/ polyester braid?

Ans): Actually it is a fibre lapping on the copper conductor (I mentioned braiding to make the language simpler; since braiding process is much more commonly done than lapping). The lapping gives 100% coverage, though the surface of lapping is not 100% smooth. But since the conductor size is 7/0.15mm, we get alomost a round cross-section after the lapping.

- A single layer of helically served polyester insulated fiberglass cored fiber with an outside diameter of 0.10 mm (0.0039 inches)

The Overall Served Diameter is 0.65 mm (0.0256 inches)

Secondary Insulation

A single layer of flexible PVC insulation, dried and pressure extruded.
The wall thickness is 0.325 mm (0.0128 inches)

The overall diameter is 1.30 mm (0.0512 inches)

OK, if I have done the above all right then the first problem is the two different thermoplastic materials (polyester and PVC) and the basic lack of adhesion between them. The secondary problem is that if you heat the primary insulation, it will start coming off (building up) in the tip (nozzle) until the conductor breaks. This is especially true if there is a diamond insert in the tip (nozzle).

Likewise, there can be no adhesion promoters applied because of the very small dimensions and tight tolerances in the tip (nozzle).

The first suggestion I have is to ensure that the polyester fibers are really clean and not contaminated with some sort of lubricant like silicone. That would be a disaster from an adhesion perspective.

After that issue is cleared, there are only a few very basic ways of trying to get more adhesion and they have to do with keeping the interface between the two plastics warm for a longer period of time.

The first is to lengthen the air gap between the crosshead and the quench point in the cooling trough and the second is to use hot water cooling or gradient cooling in the cooling trough. The third is to increase the PVC melt temperature and the fourth is to reduce the line speed but I am against reductions in productivity unless absolutely necessary.

I suggest that you do some quick experimentation with the first three of these suggestions because everything else, apart from perhaps cleanliness, seems fine.